An important aspect of modeling water quality in water distribution system (WDS) is to predict the temporal and spatial distribution of disinfectant, and the formation of disinfection byproducts (DBP). Consistent efforts have been made to investigate the kinetics of chlorine decay and trihalomethanes (THM) formation in WDS, which are caused by the reaction of chlorine with natural organic matter (NOM). NOM is a heterogeneous mixture of complex compounds. Each specific compound shows individual reactivity with chlorine. Therefore, to better understand and predict the kinetics of chlorine decay and THM formation, the core assumption of this study was established. That is, the variable reactivity of NOM should be involved into the second order kinetics model. Specifically, each single reactive site provided by NOM shows its individual reactivity towards chlorine decay and THM formation, which can be expressed by its individual reaction rate constant, while the mixture compounds of NOM shows the overall reactivity with respect to chlorine decay and THM formation, which should be expressed by an overall reaction rate coefficient. With the reaction progress, the overall reaction rate coefficient was assumed to be continuously decreasing with the reaction time due to the decreased concentration and reactivity of NOM. The decreased overall reaction rate coefficient was referred as a variable reaction rate coefficient (VRRC) in this dissertation. The VRRC was calculated as an exponential function with limited model parameters, which was only related to the characteristics of NOM but independent of chlorine concentration. By introducing the VRRC, the required model parameters were reduced, calibration was simplified and therefore the models showed abilities for a wider application. Consequently, a systematic work has been carried out to develop VRRC models of chlorine decay and THM formation based on the above mentioned assumption, and further extend and validate the models under different chlorination conditions. The following specific topics were addressed. • A VRRC model of chlorine decay was developed and validated under different conditions, including different initial chlorine dosages, different temperature, rechlorination and water mixing conditions. • Based on the identical assumption applied in the chlorine decay model, a VRRC model of THM formation was also developed and also validated under different chlorination conditions, such as, different initial chlorine dosages, changeable temperature condition and rechlorination. • The model application was extended from bulk reaction to wall reaction by considering the presence of pipe deposits in the WDS. Both the chlroine decay and THM formation models were advanced and validated when pipe deposits were present in water. • To further validate the core assumption proposed in this dissertation and also validate the proposed models have a wide application, the residual chlorine and THM concentrations in chlorinated swimming pools were predicted. Both the model accuracy and model adequacy were evaluated through statistical analysis. The results showed that the proposed models were well suited for application in water quality modeling for distribution systems.